Flip chips are monolithic semiconductor devices, such as integrated circuits, having bead-like terminals formed on one surface of the chip. The terminals, commonly referred to as bumps, serve to both secure the chip to a circuit board and electrically interconnect the flip chip's circuitry to a conductor pattern formed on the circuit board, which may be a ceramic substrate, printed wiring board, flexible circuit, or a silicon substrate. The bumps are generally formed by selectively depositing a metal on the flip chip. One known method is to selectively electroplate a solder composition on the flip chip, and then reflow the solder composition by heating the composition above its liquidus temperature so that the molten material coalesces to form the bumps on the surface of the chip. During the attachment operation, such bumps are registered with their corresponding conductors and then reheated above the solder composition's liquidus temperature in order to bond the chip to the conductors.
Alternatively, the bumps on a flip chip can be formed by a metal composition that is not reflowed during attachment. These may be formed by evaporation of the metal through a mask, or electrolessly or electrolytically plating the metal or combinations of metals. Bumps may also be formed by wire-bonding a gold wire, which has been flame reflowed, to the chip terminals, and then breaking the wire to form a quasi-spherical bump that is then flattened such that the surfaces of the terminals are planar. The preformed terminal bumps on the flip chip are then registered with conductive material that was previously deposited on each of the conductors to which the terminal bumps are to be electrically interconnected. The conductive material can be a conductive adhesive that attaches the flip chip to the circuit board and makes the electrical connection between the flip chip terminals and their conductors. Alternatively, the conductive material may be a solder composition that is selectively deposited with a stencil, screen or mask onto a portion of the conductors to form a suitable terminal pattern on the circuit board. FIG. 2 illustrates such a technique, in which a portion of a conductor 12 formed on a substrate 10 is exposed by an opening 14 in a mask 16 to form a terminal pad 24. When opening 14 is rectangular, the exposed portion of conductor 12 has a rectangular shape. The potential for misregistration of conductor 12 relative to opening 14 in mask 16 necessitates that opening 14 be large enough that the total width and sidewalls of conductor 12 are not obscured by mask 16. Consequently, portions of substrate 10 are also exposed by mask 16, as depicted in FIG. 2.
Inherently, the volume of conductive material used to secure a terminal bump of a flip chip to terminal pad 24 cannot be precisely controlled due to these same tolerances. If the conductive material is an adhesive, it is especially difficult to precisely deposit the adhesive in sufficiently-controlled volumes that will ensure attachment yet remain isolated from each other after the flip chip has been registered. Furthermore, because it is often advantageous in an electronic design to run conductor traces between adjacent chip terminals, misregistration of mask 16 can expose one of these traces in the same opening 14 as conductor 12 for terminal pad 24, resulting in a short when the conductive material is deposited in opening 14. This consequence can only be prevented by limiting the closeness of the trace to terminal pad 24, which severely limits the desirable ability to run traces between adjacent terminals pads 24. Consequently, inadequate adhesion and shorting between adjacent terminals and between terminals and their adjacent traces are significant challenges with the use of either conductive adhesives or solder to attach flip chips to circuit boards.
FIG. 1 illustrates the use of a solder composition as the conductive material for attaching a bump 22 of a flip chip 20 to a terminal pad 24. Electroplated solders are typically deposited through a mask such as that represented in FIG. 2, while solder pastes are typically deposited with a stencil or screen. With either approach, the solder composition is typically deposited onto substrate 10 in a relatively large amount to maximize volume, i.e., more than is required to cover the exposed portions of conductor 12 and substrate 10, and then heated to liquefy the solder. On liquefication, the solder coalesces to form a solder bump 18, with surface tension causing bump 18 to form on terminal pad 24 and acquire the semi-spherical shape shown. Flip chip 20 is then attached to the circuit board by registering bump 22 with solder bump 18, and then reflowing solder bump 18 to attach and electrically interconnect bump 22 with terminal pad 24.
FIG. 1 illustrates a difficulty with the use of solder bumps, in which terminal bumps 22 on chip 20 are prone to becoming misregistered with solder bumps 18 on the circuit board due to the semi-spherical shape of solder bumps 18. Specifically, terminal bumps 22 tend to slide off their solder bumps 18 due to the round shape of bumps 18 and 22, such that any further lateral movement of flip chip 20 relative to substrate 10 leads to loss of thermal contact between terminal bumps 22 and their corresponding solder bumps 18. Consequently, a flattening operation is typically necessary to provide flatter surfaces on which bumps 22 of chip 20 can be registered. The additional flattening operation is undesirable from the standpoint of processing time and costs. In addition, the use of flip chips on opposite sides of a circuit board is deterred or prevented because the flattened bumps on the side opposite to that being soldered will reflow to reacquire their preflattened semi-spherical shape.
The above-noted difficulties associated with the use of conductive adhesives and solder compositions are further aggravated by current trends in the electronic industry that impose significant size constraints to achieve smaller electronic packages. Such constraints often dictate finer pitch conductors and solder bumps than the typical 125 micrometers (about five mils) attainable by the board bumping process described above.
As can be appreciated from the above, prior art processes for attaching chips to a terminal pattern of a circuit board are rather complex and costly. In addition, precise volume control of the conductive material is difficult due to the dimensional and registration tolerances associated with masks, screens and stencils used to deposit such materials. The net effect is that the spacing between terminals must be much larger than is desired, such that the above flip chip attachment techniques cannot be used with many otherwise applicable integrated circuits having a pitch smaller than about 0.4 millimeters (about sixteen mils). Finally, imprecise volume control of the conductive material increases the likelihood of inadequate adhesion if insufficient material is deposited, or shorting between terminals if excessive material is deposited.